Key Concepts of Section 20.3

The Extracellular Matrix I: The Basal Lamina

• The matrisome is the ensemble of proteins that compose the ECM itself and associated proteins that covalently modify (e.g., chemically cross-link, phosphorylate, cleave) the ECM.

• The basal lamina, a thin meshwork of ECM molecules, separates most epithelia and other organized groups of cells from adjacent connective tissue. Together, the basal lamina and the immediately adjacent collagen network form a structure called the basement membrane.

• Four ECM proteins are found in all basal laminae (see Figure 20-23): laminin (a multi-adhesive matrix protein), type IV collagen, perlecan (a proteoglycan), and nidogen/entactin.

• Adhesion receptors such as integrin anchor cells to the basal lamina, which in turn is connected to other ECM components (see Figure 20-1). Laminin in the basal lamina is the principal ligand of α6β4 integrin (see Table 20-4).

• Laminin and other multi-adhesive matrix proteins are multidomain molecules that bind multiple adhesion receptors and ECM components.

• The large, flexible molecules of type IV collagen interact end to end and laterally to form a mesh-like scaffold to which other ECM components and adhesion receptors can bind (see Figures 20-23 and 20-26).

• Type IV collagen is a member of the collagen family of proteins, which is distinguished by the presence of repeating tripeptide sequences of Gly-X-Y that give rise to the collagen triple-helical structure (see Figure 20-25). Different collagens are distinguished by the length and chemical modifications of their α chains and by the presence or absence of segments that interrupt or flank their triple-helical regions.

• Perlecan, a large, multidomain, secreted proteoglycan that is present primarily in basal laminae, binds many ECM components and adhesion receptors. Proteoglycans consist of membrane-associated or secreted core proteins covalently linked to one or more specialized polysaccharide chains called glycosaminoglycans (GAGs).